Cordierite and aluminum titanate-based substrates, typically in the form of a honeycomb body, have been used for a variety of applications, such as catalytic substrates and filters for diesel particulate emission control. In order to respond to increasingly stringent emission standards for light- and heavy-duty vehicles, the substrate materials must be highly porous to allow gas flow through the walls without restricting the engine power, and must show high particulate filtration efficiency, while inducing minimum pressure drop. Such substrates must also withstand the corroding environment and be able to withstand thermal shock during rapid heating and cooling. Cordierite has low thermal expansion and is therefore suited for applications where high thermal shock resistance is required. A cordierite crystal shows anisotropy in its thermal expansion with a negative coefficient of thermal expansion in its crystallographic c-axis and positive thermal expansion for its a-axis and b-axis, which creates a build up of local stresses in a polycrystalline cordierite material during thermal cycling. For sufficiently large grain or domain size, e.g., agglomerates of crystals with similar orientation, those stresses lead to microcracking during cooling. Several features in the microstructure contribute to the stress leading to microcracking in cordierite ceramics: texture, such as grain/domain size and local misorientation with the largest effect for misorientational tilt in the a/c crystallographic directions or the presence of second phase intergranular glass films or precipitates having different expansion. Microcrack network densities in commercially available as-processed cordierite honeycomb substrates extend over a wide range.
Microcracks form and open during cooling and close again during heating. During thermal cycling, microcracks cyclically form, open and close; and in the ideal case, the microstructure of the material does not evolve. In reality, however, many cordierite ceramics suffer changes in their microstructure with an increasing number of thermal cycles, particularly in their microcrack density. Thermal shock does not only induce reversible microcrack formation and opening, but can also lead to growth of existing microcracks. It becomes increasingly difficult to fully close large microcracks during heating, especially if the temperature is not high enough and the hold time at temperature is not long enough for healing the cracks, as usually experienced by a filter during operation in a car. Thus, growth of the microcrack network can occur during thermal cycling. Failure of the honeycomb parts during operation results from formation of macrocracks and their propagation through the part. For microcracked ceramics, such macrocracks most likely form from interconnection and growth of microcracks. Thus, durability during operation of a honeycomb substrate or filter repeatedly exposed to thermal cycling can be limited by the instability of the microcrack network during cycling; its extension will diminish the strain tolerance of the part and ultimately lead to fracture and failure of the part.
Microcracking not only helps to lower the thermal expansion of the cordierite ceramic, typically from CTE=17×10−7 K−1 for non-microcracked porous cordierite to a range from 1 to 5×10−7 K−1, it also reduces the ceramic's strength. Initial thermal shock resistance of microcracked cordierite-based materials is often evaluated from the material's strain tolerance and its thermal expansion by using a factor TS, defined as TS=500+σ/E-mod*CTE500° C.-900° C. with σ being the strength, E-mod the elastic or Young's modulus and CTE500° C.-900° C. the average thermal expansion coefficient from 500° C. to 900° C. The TS factor provides an indication of the thermal shock temperature in ° C., the honeycomb part is expected to withstand. For many cordierite honeycomb materials, this factor is between TS=900° C. and TS=1100° C., suggesting that filters made of those materials should withstand thermal shock up to about 900° C. to 1100° C. However, thermal cycling experiments of (bare) honeycomb parts do not always follow the prediction of the TS factor of the as-processed materials.
A material-dependant evolution of the microcrack density during thermal cycling or during filter operation on a car can occur. This evolution is strongly dependant on the ceramic material. Some materials do not show any significant evolution in their microcrack network density, others show a strong increase during extensive thermal cycling. A low initial microcrack density is not necessarily a guaranty of good longtime performance. A material with very low initial microcrack density and high initial strain tolerance can undergo during thermal cycling a strong degradation of its thermomechanical performance due to extended microcrack formation and growth, while a material with an initially rather high microcrack density may have a stable microcrack network and preserve its initial thermal shock resistance during severe cycling. These examples are meant to illustrate that a material's initial thermomechanical performance can degrade during thermal cycling and is not a sufficient indicator of long time performance under thermal cycling. For high durability of a filter during thermal cycling application, high fracture toughness of the material is needed. High fracture toughness restricts (or in the best case even inhibits) crack formation and propagation of the microcracks under thermomechanical stress, avoids fatal microcrack growth and helps to preserve the initial thermomechanical filter performance.
Although the stability of the microcrack network during thermal cycling is crucial for durability of ceramic filters and substrates in automotive exhaust applications, the focus on substrate and filter material development rarely centers on fracture toughness.
From a fundamental point of view, the particularities of the microcrack network in microcracked aluminum titanate-based and cordierite-based ceramics and the resulting limitations in fracture toughness during repeated thermal cycling of such ceramics have not been carefully addressed in the past and still lack specific material engineering approaches to overcome these limitations.
New products that will meet the key desired filter properties: high initial thermal shock resistance, high filtration efficiency and low pressure drop and offer in addition higher durability through an improvement in fracture toughness are desired and will enable preparation of products with higher thermal shock tolerance and improved lifetime.
The art lacks suitable dispersion-toughened microcracked ceramics having improved durability during thermal cycling. The present invention is directed to overcoming these and other deficiencies in the art.